Over the years, cruise control has minimized fuel consumption inefficiencies that result from oscillations around the cruising speeds for the typical driver. However, this improvement is mostly realized in flat terrains and at ideal speeds such as 55 mph highway speeds. However, experienced drivers are capable of higher fuel efficiency rates than state of the art cruise control systems. This is especially true for hilly routes. In fact, there is a large community of “hyper-milers” who compete against each other to get the best fuel economy from their vehicles. There are several reasons why this is possible:
Advanced drivers use their vehicle's inertia and the corresponding kinetic and potential energy. They use kinetic energy in the body of the vehicle to aid areas where more energy is needed such as hill climbing. In many cases, a driver may accelerate on flats before the hill to slowly accumulate the energy that will be consumed as the vehicle climbs the hill as this may be advantageous to the fuel economy of the vehicle. Likewise, they may be more likely to let the vehicle reach higher speeds at the end of a downhill, rather than breaking or using much less throttle and “coast” on the flat after the hill is has flattened out.
Advanced drivers tend to drive faster on downward slopes and slower on up-hills therefore reducing changes in throttle and thus reducing changes in engine power output.
As the vehicle goes uphill using a standard cruise control, the transmission sometimes downshifts to a lower gear. Even though a cruise control may provide hysteresis to reduce this effect, the standard cruise control is not aware of the elevation profile ahead and therefore cannot select the gear and throttle position for optimal fuel efficiency.
The fuel efficiency as a function of speed in a particular vehicle changes depending on a variety of factors which include: RPMs, gear, aerodynamic drag, rolling resistance, and the load that the engine is being subject to. As a driver sets the speed in the cruise control, this speed will seldom be the optimal. And more importantly, there is not a single speed that will maintain the vehicle at an optimal fuel economy given a particular elevation profile. It may sound counterintuitive, but under certain conditions, faster may be more fuel efficient. This is clearly illustrated at very slow speeds where the fuel efficiency increases significantly from parked, where fuel efficiency is 0.